The overall objective of the proposed research is a quantitative description of the life history of messenger RNA populations in several types of animal cells. Specific aims are: 1) Kinetic analyses of RNA synthesis in L cells, including rates of chain elongation, number of nascent RNA molecules per cell, and rate of processing of nuclear RNA into messenger RNA. A new technique for rapid equilibration of the ATP pool in cells makes these measurements possible. 2) A test of the hypothesis that a process of post-transcriptional RNA selection exists, which results in the translation of some mRNA species and the degradation of others without translation. This experiment will employ DNA copies of mRNA's from mouse tissues. The complementary DNA will be used as probes to detect mRNA sequences among the nuclear RNA's from various tissues. 3) An analysis of the abundance classes of functional mRNA in L cells and sea urchin embryos. A series of RNA/DNA hybridizations under conditions of RNA excess will be used to determine whether some mRNA's are more abundant than others.